Mammalia: Visceral Systems 659 



organs of all Amniota develop as modifications of the wall of the 

 cloaca. In Chelonia and Crocodilia, the midventral region of the 

 cloacal wall contains a longitudinal thickening consisting of spongy 

 and highly vascular connective tissue, more or less definitely divided 

 into right and left parts, the corpora cavernosa, underlaid by a more 

 compact corpus fibrosum. Extending along the internal ventral 

 surface of the cloaca and between the two cavernous bodies is a median 

 seminal groove. Posteriorly, these structures continue into a tongue- 

 shaped flap (glans penis) which projects freely above the cloacal 

 floor, pointing backward. The corpora cavernosa, when distended 

 by blood, arch over the groove and convert it into a tubular passage, 

 the whole structure meanwhile becoming so greatly distended that it 

 protrudes through the external cloacal aperture. The spermatic fluid, 

 discharged from the more anterior region of the cloaca, is conducted 

 outward through the temporarily tubular seminal passage. 



In mammals the copulatory organs are more highly differ- 

 entiated than in other amniotes, and most so in placental mammals. 

 At an early stage in a placental embryo, but after the primary cloaca 

 has been horizontally divided into a rectal and a urinogenital part 

 (Fig. 494), the latter part enters upon a somewhat complex set of 

 developmental changes. At a median spot situated in its postero- 

 ventral wall and at the ventral edge of the cloacal membrane (the 

 wall consisting of ectoderm externally, the cloacal endoderm internally, 

 and mesenchyme between), rapid growth sets in, especially in the 

 mesenchyme, producing at first a mere roundish, knoblike, solid body 

 which projects externally — the genital prominence or tubercle 

 (Fig. 498A). This prominence then elongates more or less rapidly — 

 more so in the male, less in the female — to become a projecting cylin- 

 dric structure, the phallus, bearing an enlarged solid distal end (Fig. 

 498B). Meanwhile, the urinogenital cloacal cavity perforates the 

 cloacal membrane and acquires a slitlike external aperture at the base 

 of the phallic outgrowth. This aperture becomes continuous with a 

 deep median groove formed along the dorsal surface of the phallus — 

 the urinogenital or urethral groove. The longitudinal lips of this 

 groove become much thickened, appearing then (viewed externally) as 

 the so-called "internal genital folds" (Fig. 498B, C). At either 

 side, right and left, of the phallus develops a broad swollen region 

 variously known as a genital ridge or swelling, scrotal fold, or 

 external genital fold (Fig. 498C, s). 



These several structures attain maximum development in the male. 

 Continued elongation of the embryonic phallus forms the shaft of 

 the penis while the distal tubercle of the embryo becomes the enlarged 

 terminal glans penis (Fig. 492). The urethral groove eventually 



